1. Field of the Invention
The invention concerns liquid crystal (LC) compositions, such as they are used for electrooptic displays with a twisted nematic liquid crystalline phase (so-called twisted nematic displays or turned nematic cells).
2. Description of the Prior Art
Twisted nematic displays are known in the prior art, see M. Schadt and W. Helfrich, Applied Physics Letters, 18/1971/127. They are prepared on a large technical scale especially for digital displays, for example, in watches, small computers and the like.
A "liquid-crystal" is used as dielectric between the electrodes of such display cells, i.e., a substance which, during the temperature-dependent solid/liquid phase transformation, forms at least one intermediary liquid phase (mesophase), in which the molecules are oriented, and therefore arranged relatively to each other. This arrangement of molecules of a liquid mesophase causes the electric and optic manifestations of anistropy which are required for the operation of the liquid crystal displays.
For liquid crystal displays with a twisted nematic phase, liquid crystal substances with a nematic orientation of molecules in the desired temperature range and with a positive dielectric constant anistropy are required. In such substances, the dielectric constant of the liquid crystal parallel to the molecular axis (.epsilon..sub..parallel.) must be larger than the dielectric constant vertical to the molecular axis (.epsilon..sub..perp.). Dielectric constants may, for example, be measured according to the method described in the Z. Naturforschung (J. of Physics) 29a (1974) 905. The magnitude of the positive value of the difference .epsilon..sub..parallel. -.epsilon..sub..perp. is the measurement for the positive dielectric constant anisotropy and is generally symbolized by .DELTA..epsilon..
For the operating capacity of the liquid crystal displays it is also important that the liquid crystalline mesophase be maintained underall temperature conditions that can normally be expected, typically at temperatures in the range of 0.degree. C. or less, up to 65.degree. C. or more. The lower limits of this range are determined by the temperature of the transformation crystalline/nematic-liquid crystalline (C/LC). The higher limits are determined by the temperature of the transformation nematic-liquid crystalline-isotropic-liquid (LC/LI or clarification point).
Finally, the liquid crystalline substance in the anisotropic liquid range must have a sufficiently low viscosity and must niether show undesired chemical or physical changes. These properties must also include the physical characteristics of the mixture system under the operational conditions of the cell. It is particularly important that the properties hold under the characteristics of the liquid crystal substance when in the form of a thin layer of a thickness of about 10 micrometers.
Up to now, no single chemical compound has become known which optimally fulfills all these criteria. The technically usable liquid crystal compositions are mixtures of various substances (see, for example, DE-OS Nos. 2,321,632; 2,450,088 and 2,502,904 as well as U.S. Pat. No. 3,951,846). The significant requirement with respect to a liquid crystal composition which meets the above-mentioned criteria, that the mixture be as simple as possible, i.e., that it be composed of few compounds, can be met according to a proposal of the applicant (Swiss Patent Application No. 15,978/77), by means of mixtures which contain at least a first compound of the formulas (10), (11) ##STR3## in which R' is an alkyl-group with 1 to 10 C-atoms in an essentially straight chain; R" is an alkyl- or alkoxy-group with 1 to 10 C-atoms in an essentially straight chain and m is two or three, together with at least a second compound of the formula (12) ##STR4## wherein H denotes that the six-membered ring is fully hydrogenated; in which R"' is equivalent to R' and n is one or two; and also a third compound which corresponds to one of the formulas (10), (11) or (12), but which is different from the first as well as from the second aforementioned compounds. Suitable compounds of the formulas (10), (11) and (12) are known.
Another proposal of the present applicant (Swiss Patent Application No. 5548/76) is to use as admixture for the increase of the positive dielectric constant anistropy of liquid crystal mixtures, new compounds of the formula (13) ##STR5## in which X' is hydrogen or the formyl group, R.sup.4 is an alkyl group with up ro 8 C-atoms, Y' is a single bond between both rings A and B or Y' is a divalent organic residue selected from the group of azomethin-, carbonyloxy- and phenylene residues; and Z' is a monovalent, electron-withdrawing residue as, for example, a nitrile group, and wherein the rings A and B may be substituted, if necessary.
As was described in greater detail in the Swiss Patent Application No. 5548/76, herein incorporated by reference, liquid crystalline characteristics of known compounds carrying the dimethylamino group instead of the group ##STR6## which is characteristic for the new compounds, were not determined. The new compounds (13) showed unexpected liquid crystalline or potential liquid crystalline characteristics and were therefore much better suited as admixture for an increase in .DELTA..epsilon..
A need therefore continues to exist for liquid crystalline compounds which can form either singly or jointly liquid crystalline compositions with a wide temperature range, a low viscosity, a high positive dielectric anistropy, and a high anistropy of the refractive index.